1. Field of the Invention
The present invention relates to encoding and decoding of digital communication systems. More particularly, the present invention relates to multi-level encoding and decoding digital bits using the same or an identical encoder and decoder.
2. The Prior Art
Generally, communication systems include both a digital signal transmission and receiving system. The transmission portion of the digital signal transmission and receiving system includes an encoder, possibly a puncturing or zero-tailing module, and a modulator, providing a modulated signal at a communication channel. Similarly, a receiving portion of the digital signal transmission and receiving system includes a demodulator, possibly a depuncturing or block decoder module if punctured or zero-tailed, respectively, during encoding, and a decoder.
The modulated signal includes an in-phase component and a quadrature component. When the modulated signal is received, after conversion from an analog to a digital signal, each bit is demodulated into the in-phase and quadrature signal components by the demodulator using sine and cosine functions. The decoder may comprise a Viterbi decoder, which may be used to decode these convolutional codes.
In satellite broadcasting, severe location-dependent fades can occur due to rain attenuation. If a single trellis coded modulation (TCM) scheme is used, then those locations with severe attenuation will loose the incoming signal, as shown in FIG. 1a. On the other hand, by using different TCM schemes, the digital signal can be encoded in such a way that information at a lower rate is received under severe attenuation. Receivers in locations with less adverse atmospheric conditions can receive information at higher rates, as shown in FIG. 1b. This is known as graceful degradation. However, to achieve an efficient system design, different constellations may need to be used in multiplexing TCM schemes. Thus, a control mechanism to switch seemlessly between different rates and constellations is required. This leads to a reduced throughput, due to overhead information, as well as a more complex receiver structure to process different signal constellations and to synchronize properly.
What is needed is a method which would work over the same constellation without degrading the signal under adverse atmospheric conditions; thereby, eliminating the overhead associated with multiplexing control and/or rate switching.
The purpose of the present invention is to provide a method and apparatus for encoding and decoding digital signals which would allow the system to compensate for atmospheric degradation using a multilevel, j, encoding/decoding system where each level uses either the same or an identical encoder/decoder. This would allow for higher bandwidth efficiency and would simplify the structure of the receiver.
The encoding method includes the steps of: choosing a component code rate, rj, for each substream, splitting the data stream into a plurality, j, of data substreams, sending each substream through an identical encoder; possibly puncturing or zero-tailing each data substream, combining the substreams into symbols of n output length, applying a 2j symbol generator that provides an output of a plurality of modulator values, applying these modulator values to a modulator for transmission over the channel. The multi-stage decoding method includes the steps of: receiving the multi-level encoded, modulated signal, pre-processing the data including depuncturing or block decoding if the data was punctured or zero-tailed respectively during encoding, sending the data through an identical decoder, saving the first-level information bits in a buffer, and repeating the process of pre-processing, decoding, storing, and delivering the data for j-stages. Additionally, using a nonstandard partitioning scheme, encoded data can be mapped into disjoint regions of X-dimensional space, creating independent variables that can be identified and eliminated from decoding schemes.